1. Field of the Invention
The present invention relates in general to generating bubbles for friction-reducing ships and lowering of skin-friction, and relates in particular to a technique of jetting micro-bubbles from a ship to reduce skin-friction.
2. Description of the Related Art
Technologies for reducing skin-friction effects of ships are disclosed in Japanese Patent Applications represented by First Publication numbers: S50-83992; S53-136289; S60-139586; S61-71290; as well as in Japanese Practical Utility Models, S61-39691; and S61-128185. These friction-reducing ships produce lower skin-friction at cruising speeds by the action of numerous bubbles intervening at the interface between water and hull, produced by jetting gases such as air from the hull surface into the water.
To increase the friction reduction effects at the hull interface using such bubble jet techniques, it is known that, in a turbulent boundary layer formed near the hull surfaces, a constant .eta..sub.m in the following expression (1) must be made large: EQU .eta..sub.m =.lambda..sub.m /d.sub.b .multidot..alpha..sub.m.sup.2/3(1)
where .alpha..sub.m is an average void fraction in a turbulent boundary layer; d.sub.b is a diameter of micro-bubbles; and .lambda..sub.m is a constant related to the size of a ship.
Although it may be thought that the volume of gas blowing into the water should be made higher to increase the average void fraction .alpha..sub.m, but it is found that higher gas flow rates cause micro-bubbles to be pushed out of the turbulent boundary layer, resulting in no increase in the average void fraction .alpha..sub.m. Also, there has not been an effective measure for gauging the magnitude of average void fraction .alpha..sub.m.
In the current technologies cited above, micro-bubbles are produced by jetting compressed air from compressor through a plurality of holes or porous plate. However, it has been known that the diameter of micro-bubbles produced by jetting air are governed by the bubbly flow speed (approximately equal to the ship speed), it is impractical to restrict the ship speed to suit the bubble size, and it would be ideal to control jetting parameters to optimize the bubble size for any speed of the ship.